Compensated time division fm stereo multiplex system



June 9, 1964 R. J. HIRVELA ETAL 3,136,860

COMPENSATED TIME DIVISION FM STEREO MULTIPLEX SYSTEM Filed Aug. 1, 1962 2 Sheets-Sheet 1 5 LEFT 7 a HIGH 9 ,/0 J cHANNELa R MPHA KC EMITTER f g f g P EE I'WORK fl" 'Tf FILTER FOLLOWER} o E MULTIPLEX h KC COMPOSITE FILTER OUTPUT SIGNAL RIGHT //6 J CHANNELBPRE-EMPHASIS 0 l5 KC EMITTER AUDIO NETWORK I" 55% FILTER FOLLOWER FIG FIG: 2a SkN I J ED A T S B KC RIGHT CHANNEL F/G. 2b SAMPLED AT aaxc CYCLE LATER 57 2 COMPOSITE SIGNAL (AUDIO ONLY PRESENT) Attorneys June 9, 1964 Filed Aug.

R. J. HIRVELA EI'AL 3,136,860

COMPENSATED TIME DIVISION FM STEREO MULTIPLEX SYSTEM 1, 1962 2 Sheets-Sheet 2 312 Mm; SAMPLE, WMFORM AFTER FILTERING LEFT AUDIO COMPOSITE SIGNAL AFTER FILTERING AND AMPLITUDE CORRECTION 4a LEFT CHANNEL SAMPLED AT 38KC RIGHT CHANNEL 40 SAMPLED AT 38KC CYCLE LATER 4 COMPOSITE SIGNAL BEFORE FILTERING COMPOSITE SIGNAL 4d AFTER FILTERING (DOUBLE SIDEBANDS ONLY PRESE CENTERED AT 38KC) /NVEA/T0 R.S' Robert I/. I H/rve/a Frank D. M LI'n Afforneys United States Patent COMPENSATED TiME DIVISION FM STEREO MU LTIPLEX SYSTEM Robert J. Hirvela and Frank I). McLin, Cedar Rapids, iowa, assignors to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Filed Aug. 1, 1962, Ser. No. 214,074 3 Claims. (Cl. 17915) This invention relates to a time division multiplexing system and more particularly to a stereophonic signal generating system capable of producing a composite signal having a first component that is the sum of two audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency, the modulation of which is the difference between said audio frequency signals.

A broadcasting system capable of approaching the reproduction of sound exactly as it would be heard directly from the point of origination has been under study for some years. It has been found that a reasonable approximation to the above-stated end can be achieved through the stereophonic effect gained by the use of left and right microphones if the original frequencies, as received at the microphones, are preserved intact during transmission and then properly fed to left and right speakers. The degree of success in preserving the two audio signals (received at the left and right microphones) intact during transmission determines, at least in part, the quality of the stereophonic system.

During transmission, both time delay and amplitude differences between the two audio signals must be preserved, as well as adequate channel separation, to achieve a good stereophonic effect. Although maintaining time delay differences in transmission did not prove to be a serious problem in some prior systems, maintaining amplitude differences has proven to be more difficult, as has maintaining adequate channel separation.

Heretofore, channel separation in stereophonic systems has usually been achieved by use of a matrixing technique wherein the left and right audio signals Were added to produce a sum signal (L+R) and subtracted to produce a difference signal (L-R). The sum signal and the difference signal were then transmitted over two transmission channels. To recover the original left and right audio signals at the receiver, (L-l-R) was added to (LR) to produce 2L and subtracted to produce 2R. A disadvantage of the matrixing technique, however, is that the gain and phase shift between the two channels must be maintained within close tolerances to maintain channel separation throughout the system.

It is a feature of this invention that a time division multiplexing technique is utilized. While the matrixing system inherently provides a two wire output (for two channel transmission), the time division technique provides a composite stereophonic signal as one output and hence requires only single channel transmission.

Another advantage of the time division multiplexing technique over the matrixing technique is that the transformers, or their equivalents, necessary for matrixing have been eliminated (as brought out more fully hereinafter, the only transformer used for the time division technique as utilized in this invention being for coupling the superaudio switching signal to the diode gate).

It is therefore an object of this invention to provide a time division multiplexing system capable of producing a composite signal from two input audio frequency signals that is suitable for single channel transmission yet adequately maintaining channel separation and amplitude differences so that said audio frequency signals may be recovered substantially intact after transmission.

It is another object of this invention to provide a time division multiplexing system capable of producing a composite signal having a first component that is the sum of two audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency, the modulation of which is the difference between said audio frequency signals.

It is still another object of this invention to provide a stereophonic signal generating system wherein left and right audio frequency signals may be coupled to gate means switched at a superaudio frequency rate whereby the output produced after filtering is a composite signal having a first component that is the sum of said left and right audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency, the modulation of which is the difference between said left and right audio frequency signals.

With these and other objects in view which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination and arrangement of parts substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the herein disclosed invention may be included as come within the scope of the claims.

The accompanying drawings illustrate one complete example of the embodiment of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

FIGURE 1 is a schematic and block diagram of the stereophonic generator, or time division multiplexing system, of this invention;

FIGURE 2 is a series of wave forms illustrating typically how a composite signal is produced under conditions where the left channel inputsignal equals the right channel input signal; 7

FIGURE 3 is a series of Wave forms illustrating typically how the composite signal is produced under conditions Where an audio signal is received only at the left input channel; and

FIGURE 4 is a series of wave forms illustrating typically how the composite signal is produced under conditions where the right channel input signal is equal to minus the left channel input signal.

Referring now to the drawings in which like numerals have been used for like characters throughout, the numeral 5 indicates generally the stereophonic signal generating system (or time division multiplexing system as it may be more broadly termed) of this invention including time division multiplexing unit 6. As shown, a left channel audio frequency input signal is provided which input may be, as is conventional, through a microphone 7, a conventional pre-emphasis network 8 microsecond pre-emphasis, for example) a conventional high pass filter 9, a conventional 0-15 kilocycle (kc.) filter 1t) and a conventional emitter follower 11.

In like manner, a right channel audio frequency input signal is provided through a microphone 12, a conventional pre-emphasis network 13 (75 microsecond preemphasis, for example), a conventional high pass filter 14, a conventional 0-15 kc. filter and a conventional emitter follower 16.

The outputs from the emitter followers are coupled to time division multiplexing unit 6. The output from the time division multiplexing unit is coupled to 0-53 kc. filter 18, the output of which may be coupled to a stereophonic exciter (not shown) for transmission purposes.

Time division multiplying unit 6 includes a diode gate 26), which gate includes diodes 21, 22, 23 and 24. Diodes 21 and 22 have unlike poles (cathode of diode 21 and sasasoo anode of diode 22 as shown in FIGURE 1) connected to receive the output from emitter follower Ill through resistor 27 (in series with diode 2f), resistor 28 (in series with diode 22) and potentiometer 29, the variable tap 3d of which is connected to emitter follower II.

In like manner, diodes Z3 and 24- have unlike poles (anode of diode 23 and cathode of diode 24) connected to receive the output from emitter follower 16 through resistor 33 (in series with diode 23), resistor 34 (in series with doide Z4), and potentiometer 35, the variable tap 36 of which is connected to emitter follower 1d.

The diodes of gate 2% are alternately biased to conductance and nonconductance conditions by a conventional 38 kc. frequency signal source 40 through a transformer 4.2, the primary winding 43 of which is connected to source 40. The secondary winding 24 of transformer 42 has one side 45 connected to the anode of diode and the cathode of diode 23, while he other side side is connected to the cathode of diode 2-2 and the anode of diode 2 With this arrangement, diodes 21 and 22 are paired for simultaneous conduction as are diodes 23 and 24, each pair of diodes being repeatedly switched each one half Cycle by the superaudio frequency source 4-9.

The output from multiplexing unit 6 is coupled to filter 18 by means of lead 48 connected to center tap 49 of the secondary of transformer 42. In addition, a resistive bypass circuit SI couples the left and right input signals from the emitter followers directly to filter 18 through resistor 53 (connected to emitter follower II), resistor 54 (connected to emitter follower 16), and variable resistor 55.

Resistors 27, 2.8, 33 and 34 act to reduce distortion due to the nonlinearity characteristics of the diodes, while Variable potentiometers 29 and 35 are adjusted to balance out the fundamental and second harmonic of the switching signal supplied by 38 kc. frequency generator Resistive bypass circuit 51 adds a correction factor to the sum audio signal (L-l-R) at filter 18, this factor being 4/ 1r for purposes as brought out more fully hereinafter.

In operation, when left and right audio frequency signals are received at the left and right channel audio inputs, these signals are coupled through the pro-emphasis networks, filters and emitter followers to time division multiplexing unit 6 and more particularly to diode gate means 28 therein. Since diodes 21 and 22 are conductive simultaneously, as are diodes 23 and 24, and since the condition of each diode is caused to change every half cycle of the superaudio frequency generator 40, as brought out hereinabove, the gate alternately passes left and right audio signals at the supcraudio frequency rate (38 kc. rate as shown in FIGURE 1).

Thus, as shown in FIGURE 1, if the instantaneous voltage at side 45 of secondary winding 44 of transformer 42 is positive and the voltage at side 46 therefore negative, diodes 21 and 22 will be conductive while diodes 23 and 24 will be nonconductive. Of course, during the other half cycle, diodes 2i and 22 are switched to the nonconductive condition while doides 23 and 24 are switched to a conductive condition.

The switching operation, as described hereinabove, generates a signal, readily shown by Fourier analysis, to be equal to:

(LR) cos W t where L is equal to the left channel audio input signal; R is equal to the right channel audio input signal; W is equal to 211" (38 kc); and

.t is equal to time.

Filter 18, however, removes all terms above and including cos 3W so that out of the filter, without correction, the composite signal is equal to:

The portion of the signal bypassing gate 20 through resistive bypass circuit 51 adds the sum of the input signals (L-i-R) as a correction factor. This correction factor, as brought out hereinabove, is adjusted to be equal to 4/1r. The resulting signal from the filter with the correction factor is therefore equal to It is therefore readily seen from the above that the composite output from the system does include, in fact, one component that is the sum of the input signals (L+R) at audio frequencies and another component that is a double sideband amplitude modulated signal centered at a superaudio frequency (38 kc. as shown in FIGURE 1 and as referenced in the equations), the modulation of which is the difference between the input signals (LR). Thus, if the sidebands occupy a 15 kc. band in each direction, the double sideband signal occupies a band only between 23 kc. and 53 kc., which band is well above the audio frequency range. From the foregoing, it can be readily seen, of course, that the superaudio switching frequency chosen might be any frequency at least twice that of the highest audio frequencies expected. This invention is therefore not meant to be limited to the use of a specific 38 kc. switching frequency.

The wave forms of FIGURES 2 through 4 illustrate the desirable composite signal generated with typical left and right channel inputs. Assuming that the left and right audio inputs are equal, the composite signal to be transmitted should have only the audio portion, since with L and R equal, (L-l-R) equals two (considering each of unity strength), while (LR) equals zero. This is shown as having been carried through by FIGURE 2. FIGURE 2a shows the left channel sampled at the 38 kc. rate while FIGURE 2b shows the right channel sampled at the same rate one-half cycle later. The output, as shown by FIGURE 20 is a composite signal with only the audio component present.

Assuming that a signal is to be applied to the left channel only, that is with right channel input equal to zero, the requirement is that (L+R) equals one and that (LR) also equals one (again considering the input signal to be of unity strength). The composite signal to be produced must then consist of an audio component and a double sideband component that are equal in amplitude. This is shown to be the case by FIGURE 3. FIGURE 3a shows the left audio input signal (right is zero). This sampled wave form is as shown in FIG- URE 3b after being switched at the 38 kilocycle rate.

Since the switch, as brought out hereinabove, causes an output having the audio component plus a double sideband component centered on the switching frequency, the output after filtering but without amplitude correction (supplied by bypass resistive network 51) is as shown in FIGURE 3c.

Due to the fact that the fundamental component of a square Wave is times the square wave amplitude, the double sideband component is larger than the audio component. The

amplitude of the audio component, however, is then increased by by the correction factor. The resulting signal from the filter with the correction factor added, is, as shown as in FIGURE 4d, a composite signal wherein the double sideband amplitude is equal to that of the audio amplitude.

Considering yet another possibility, when the right channel input equals minus the left channel input, or (L+R)=zero and (L-R) =two, the composite signal has only the superaudio component. This is shown to be the case by FIGURE 4. FIGURE 4a shows the left channel input sampled at the 38 kc. rate, while FIG- URE 4b shows the right channel input sampled at the same rate one-half cycle later. When the two are added at the multiplexing unit, the resulting composite signal (before filtering) is as shown in FIGURE 40. After filtering and with the correction factor added, the resulting composite signal is shown in FIGURE 4d with only the double sideband superaudio frequency component present to the exclusion of an audio frequency component.

Particular component values which have been successfully used in a working embodiment of the multiplexing unit are as follows:

Diodes 21-24 1N270 Resistors 27-28 "ohms" 464 Potentiometer 29 -do 0-10 Resistors 33-34 do 464 Potentiometer 35 do 0-10 Resistors 53-54 do 1961 Resistor 55 do.. 0-250 It is to be appreciated, of course, that the foregoing particularized list of components is for illustrative purposes only and that the invention described herein is not meant to be limited thereto.

In view of the foregoing, it should be evident to those skilled in the art that the stereophonic signal generating system of this invention, though relatively simple, is nevertheless capable of, and well suited for, providing a composite signal output suitable for single channel transmission, which signal has a first component that is the sum of two audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency, the modulation of which is the difference between said audio fiequency signals.

What is claimed as our invention is:

l. A time division multiplexing system, comprising: first input means for receiving a first audio frequency signal; second input means for receiving a second audio frequency signal; first and second diode gate means connected to said first and second input means, respectively; a substantially constant superaudio frequency signal source; means for coupling said superaudio frequency signal source to said diode gate means to control con- (111033 11 of the same whereby said first and second diode gate means are repeatedly switched to alternately pass said first and second audio frequency signals at said superaudio frequency rate; resistance means connected in parallel with said first and second diode gate means; and means including a filter connected to receive the output from said diode gate means and to said resistance means, said filter passing an output from said system comprising a composite signal having a first component that is the sum of said audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency the modulation of which is the difference between said audio frequency signals, said resistance means in parallel with said diode gate means adding an additional audio frequency signal so that the amplitude differences between said components of said composite signal are substantially eliminated.

2. A time division multiplexing system, comprising: first and second diode gate means; a substantially constant superaudio frequency signal source; means for coupling said superaudio frequency signal source to said diode gate means to control conduction of the same whereby said first and second diode gate means are repeatedly switched at said superaudio frequency rate; first and second input means for receiving first and second audio frequency signals, respectively; first and second variable resistance means connected in series between said first input means and said first diode gate means and between said second input means and said second diode gate means, respectively, said variable resistance means balancing out said superaudio frequency signal source and the second harmonic thereof; and means including a filter connected to receive the output from said diode gate means and pass an output from said system comprising a composite signal having a first component that is the sum of said audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency the modulation of which is the difference between said audio frequency signals.

3. A time division multiplexing system, comprising: first and second diode gate means; a substantially constant superaudio frequency signal source; means for coupling said superaudio frequency signal source to said diode gate means to control conduction of the same whereby said first and second diode gate means are repeatedly switched at said superaudio frequency rate; first and second input means for receiving first and second audio frequency signals, respectively; first and second variable resistance means connected in series between said first input means and said first diode gate means and between said second input means and said second diode gate means, respectively, said variable resistance means balancing out said superaudio frequency signal source and the second harmonic thereof; resistance means bypassing said diode gate means; and means including a filter connected to receive the output from said diode gate means and connected to said bypass resistance means to pass an output from said system comprising a composite signal having a first component that is the sum of said audio frequency signals and a second component that is a double sideband amplitude modulated signal centered at a superaudio frequency the modulation of which is the difference between said.

audio frequency signals, said bypass resistance means adding to said composite signal an additional audio frequency signal that is the sum of said audio frequency signals whereby amplitude differences between said components are substantially eliminated.

References Cited in the file of this patent UNITED STATES PATENTS Eilers Dec. 25, 1962 OTHER REFERENCES 

1. A TIME DIVISION MULTIPLEXING SYSTEM, COMPRISING: FIRST INPUT MEANS FOR RECEIVING A FIRST AUDIO FREQUENCY SIGNAL; SECOND INPUT MEANS FOR RECEIVING A SECOND AUDIO FREQUENCY SIGNAL; FIRST AND SECOND DIODE GATE MEANS CONNECTED TO SAID FIRST AND SECOND INPUT MEANS, RESPECTIVELY; A SUBSTANTIALLY CONSTANT SUPERAUDIO FREQUENCY SIGNAL SOURCE; MEANS FOR COUPLING SAID SUPERAUDIO FREQUENCY SIGNAL SOURCE TO SAID DIODE THE MEANS TO CONTROL CONDUCTION OF THE SAME WHEREBY SAID FIRST AND SECOND DIODE GATE MEANS ARE REPEATEDLY SWITCHED TO ALTERNATELY PASS SAID FIRST AND SECOND AUDIO FREQUENCY SIGNALS AT SAID SUPERAUDIO FREQUENCY RATE; RESISTANCE MEANS CONNECTED IN PARALLEL WITH SAID FIRST AND SECOND DIODE GATE MEANS; AND MEANS INCLUDING A FILTER CONNECTED TO RECEIVE THE OUTPUT FROM SAID DIODE GATE MEANS AND TO SAID RESISTANCE MEANS, SAID FILTER PASSING AN OUTPUT FROM SAID SYSTEM COMPRISING A COMPOSITE SIGNAL HAVING A FIRST COMPONENT THAT IS THE SUM OF SAID AUDIO FREQUENCY SIGNALS AND A SECOND COMPONENT THAT IS A DOUBLE SIDEBAND AMPLITUDE MODULATED SIGNAL CENTERED AT A SUPERAUDIO FREQUENCY THE MODULATION OF WHICH IS THE DIFFERENCE BETWEEN SAID AUDIO FREQUENCY SIGNALS, SAID RESISTANCE MEANS IN PARALLEL WITH SAID DIODE GATE MEANS ADDING AN ADDITIONAL AUDIO FREQUENCY SIGNAL SO THAT THE AMPLITUDE DIFFERENCES BETWEEN SAID COMPONENTS OF SAID COMPOSITE SIGNAL ARE SUBSTANTIALLY ELIMINATED. 